The VF op amp is one of the most common types of op amps. High speed VF op amps are used in a variety of circuits, including arbitrary waveform generators, high linearity analog to digital converters (“ADC”), preamplifiers, digital to analog converters (“DAC”), output buffers, active filters and wireless communication receivers. In a typical VF op amp, there are two inputs, a positive input and a negative input, and one output. Across the two inputs, an error voltage (Ve) is developed. The output voltage (Vo) of the op amp is obtained by multiplying “a” by Ve, where “a” is the open loop voltage gain of the op amp. When negative feedback is applied from the output of the op amp across a resistor divider network, for example R1 and R2, to the negative input, the action of the op amp is to drive Ve to zero. This topology defines the VF op amp. Voltage feedback reduces the gain and stabilizes the VF op amp. In this situation, gain can be set by the ratio of R1 to R2. However, the gain of the VF op amp is frequency dependent. It is well known that the closed loop gain and closed loop bandwidth remain substantially constant over most of the frequencies of operation. Thus, if gain is increased, bandwidth is reduced and vice versa. As used herein, the term fast refers to the ability of the op amp to accurately reproduce high and extremely high frequency signals.
A conventional class AB op amp circuit 100 is seen in FIG. 1. This op amp circuit 100 comprises an input stage 101 with a common base gain stage 102 and a classic diamond driver 103, comprised of transistors Q22 22, Q23 23, Q24 24, and Q25 25. The front end of op amp circuit 100 comprised of input stage 101 and gain stage 102 contribute to a fast op amp configuration. Input stage 101 is fast due to the fact that the current coming out of transistors Q11 11, Q33 33, Q99 99 and Q10 10 change exponentially with the error voltage Ve. If the signal is increased then more error voltage is developed and as a consequence more current is delivered from input stage 101. Gain stage 102 is fast because it is driven by the exponential currents from input stage 101. These exponential currents increase or decrease the base to emitter voltage (VBE) on transistors Q13 13 and Q15 15. This voltage becomes a current that changes exponentially with VBE. Output stage 103 of op amp circuit 100 is a limiting factor in terms of speed of the overall circuit because when the signal being fed therein is too fast, the current from static current sources comprised of transistors Q48 48 and Q50 50 feeding the pre-drivers, comprising transistors Q22 22, and Q23 23, decreases due to parasitic capacitance (CBC and CBE) associated with transistor Q48 48 and transistor Q50 50. Less current reduces the speed of these transistors. In other words, these transistors react slower to a signal when it has less current through them. The parasitic capacitance CBC or Cμ is a capacitance caused by the depletion region in a reverse biased PN junction. The parasitic capacitance CBE or Cπ is a capacitance caused by the depletion region in forward biased PN junction. What is desired is a high speed VF op amp with a high slew rate adapted to provide full power bandwidth and lower distortion at higher frequencies. Full power bandwidth indicates how fast an amplifier is able to perform. It is a different way of measuring slew rate which is a measurement of the speed of the amplifier.